Journal of Asian Earth Sciences 113 (2015) 922–934 Contents lists available at ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes Macrostructures vs microstructures in evaporite detachments: An example from the Salt Range, Pakistan ⇑ L. Richards a, , R.C. King a, A.S. Collins a, M. Sayab b, M.A. Khan c, M. Haneef d, C.K. Morley e, J. Warren f a Centre for Tectonics Resources and Exploration (TRaX), Department of Earth Sciences, University of Adelaide, Adelaide 5005, South Australia, Australia b Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland c Karakoram International University, University Road, Gilgit, Gilgit-Baltistan, Pakistan d Department of Geology, University of Peshawar, Jamrud Road, Khyber Pakhtunkhwa, Pakistan e PTTEP, 27th Floor, ENCO Building, Soi 11, Vibhavadi-Rangsit Road, Chatuchak, Bangkok 10900, Thailand f Chevron PTTEP International Master Program in Petroleum Geosciences, Room # 239a, Department of Geology, Faculty of Science, Chulalongkorn University, 254 Phyathai Rd., Patumwan, Bangkok 10300, Thailand article info abstract Article history: The Salt Range, Pakistan is the surface expression of an evaporite detachment over which the Potwar Received 22 January 2015 Plateau fold-thrust belt has moved. Whilst previous publications regarding this region have focused on Received in revised form 31 March 2015 the petroleum prospectivity, deformation, and large-scale processes, this paper characterises the Salt Accepted 2 April 2015 Range detachment at the meso-(10 cm to 10s of metres) and micro-scale (cm to lm) and examines cor- Available online 24 April 2015 relations to the macro-scale (10s of metres to kms). Two detailed scaled cross sections are analysed alongside structural measurements to characterise the detachment at the meso-scale with optical anal- Keywords: ysis of microstructures that formed during deformation characterising the micro-scale. Both ductile and Salt Range brittle features observed in cross section indicate composite deformation processes acting simultane- Pakistan Evaporite detachment ously; this contrasts with models of salt detachments behaving homogeneously. Microstructural analysis Microstructures indicates processes of grain boundary migration and crystal lattice distortions. The microstructurally Fold-thrust belt revealed competition between intra-crystalline deformation and recrystallization at shallow depths and low temperatures links passes up-scale to mesoscale evaporite mylonites and progressively in the weaker units, whereas more brittle processes operate in the stronger lithologies in this near-unique out- crop of a the emergent toe of a major salt-bearing detachment fault. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Morley and Guerin, 1996; Rowan et al., 2004). The style of defor- mation within fold-thrust belts is strongly influenced by the nature Shortening of the Earth’s crust is commonly accommodated by of the detachment, such as strength of the overlying strata, lithol- fold-thrust belts that are mechanically decoupled from the under- ogy, pore-pressure, coefficient of friction, dip and dip direction of lying rocks by a detachment zone or horizon (Dahlstrom, 1969; the detachment (Davis et al., 1983; Jaumé and Lillie, 1988; Davis et al., 1983). The creation of fold-thrust belts is typically Dahlstrom, 1990; Koyi and Vendeville, 2003; Suppe, 2007; the result of far-field stresses, near-field stresses, or a combination Simpson, 2010). of both (e.g. King et al., 2010; King and Backé, 2010; Morley et al., The South Potwar Basin, Pakistan, is an example of a distal fore- 2011b). The driving force of deformation defines whether the land fold-thrust belt detaching over a thick salt layer, the Salt fold-thrust belt is thin-skinned, internally driven deformation, or Range Formation (Krishnan, 1966; Jaumé and Lillie, 1988; Jaswal thick-skinned where far-field stresses are dominant; a combina- et al., 1997). The fold-thrust belt is being driven by a combination tion of both is possible depending on the nature and number of of far-field stresses (continent–continent collision) and near-field active detachment layers (Morley and Guerin, 1996; King et al., stresses (gravity gliding) (Jaumé and Lillie, 1988; Davis and Lillie, 2010; Morley et al., 2011b). Detachments are typically units of 1994) and represents the southernmost expression of the overpressured shale or salt and it is the extent of these units that Himalayan orogenic deformation (Jaumé and Lillie, 1988; Davis determines the area of related deformation (Dahlstrom, 1990; and Lillie, 1994; Grelaud et al., 2002). Sediments of the Potwar Plateau are transported aseismically over the 2 ± 1° northward dip- ⇑ Corresponding author. Tel.: +61 8 8313 4971. ping basement, though local asperities within the detachment E-mail address: [email protected] (L. Richards). facilitate some seismic activity (Davis and Lillie, 1994; Satyabala http://dx.doi.org/10.1016/j.jseaes.2015.04.015 1367-9120/Ó 2015 Elsevier Ltd. All rights reserved. L. Richards et al. / Journal of Asian Earth Sciences 113 (2015) 922–934 923 et al., 2012). A down-to-the-north, normal, basement fault Here, we present structural observations and cross-sections within observed in seismic section (lines CW-13 and KK-4) causes the the Salt Range Formation and use these to characterise the Salt Range Thrust to propagate to the surface forming the epony- meso-scale structure of the detachment. Samples taken along these mous Salt Range (Fig. 1C) (Lillie et al., 1987; Baker et al., 1988). cross-sections were analysed using an optical microscope to char- Buttressing of the southward-moving allochthonous Potwar acterise the microstructural features of the detachment. We then Plateau against this fault accumulated stress at the site of this discuss the meso- and micro-scale control on the larger-scale basement fault (Davis and Lillie, 1994; Cotton and Koyi, 2000). fold-thrust belt geometry. The increased stress, distal sediment loading, and salt lubricated faulting resulted in thrusting of the Neoproterozoic to Eocene rocks comprising The Salt Range above Quaternary sediments of the 2. Geological setting Punjab Plain (Fig. 1D) (Jaswal et al., 1997; Yeats et al., 1984). Analyses of fold-thrust belts are extensively documented at the 2.1. Regional setting macro-scale (10’s of kms), in the form of regional cross-sections, seismic interpretation, satellite data, and physical and numerical The Salt Range exposes Neoproterozoic to Eocene sedimentary modelling (Lillie et al., 1987; Dirkzwager and Dooley, 2008; Chen rocks of the Salt Range, Jhelum, Nilawahan, and Chharat Groups and Khan, 2009; King et al., 2010; Morley et al., 2011b); yet few up to the Eocene Sakesar Limestone Formation, which forms the have attempted to explore the detailed structure of detachments highest peak (Fig. 1D). The interior and direct hanging-wall of at outcrop or smaller scale (e.g. Hansberry et al., 2014). Studying the Salt Range Thrust is composed of the Neoproterozoic Salt detachments at outcrop scale presents a number of difficulties; Range Formation, which forms the detachment underlying the the majority of currently active detachments occur in inaccessible associated fold-thrust belt to the north. This detachment is respon- submarine settings, whilst the few active subaerial fold-thrust sible for the observed thin-skinned contractional wedge geometry belts have insufficient degrees of erosion to allow surface outcrop in the fold-thrust belt (Lillie et al., 1987). On a larger scale, the Salt of the detachment (Hansberry et al., 2014). Our recent fieldwork, Range mirrors the geometry of the Himalayan Orogen displaying a carried out in the area of Khewra, Pakistan, has focused on the roughly E-W trend as it is a distal structure of the same orogenic structures within the Salt Range Detachment (Fig. 1A and B). event. Fig. 1. Location map, cross section, stratigraphy column. (A) Location map of the study area. (B) Geological map of the eastern Salt Range and Potwar Plateau. The dashed line AB indicates the approximate transect of the cross section in (C). MBT = Main Boundary Thrust, NPDZ = North Potwar Deformation Zone, KMF = Khari Murat Fault, SB = Soan Backthrust, SS = Soan Syncline, RF = Riwat Fault, DT = Domeli Thrust, SRT = Salt Range Thrust (after Kovalevych et al., 2006). (C) Cross section through the NPDZ, SS, and Salt Range showing imbricate stacking, pop-up and/or pop-down structures, and a thickened zone of salt above a basement fault, respectively (after Cotton and Koyi, 2000). (D) Stratigraphic column of the units within the study area (after Grelaud et al., 2002). (E) Salt Range Member subdivisions (After Sameeni, 2009 and Ghazi et al., 2012). 924 L. Richards et al. / Journal of Asian Earth Sciences 113 (2015) 922–934 2.2. Stratigraphy The presence of the Salt Range Formation evaporites has accom- modated the southward transport of the Potwar Plateau almost The stratigraphy can be segregated into four major, mechanical aseismically, while maintaining an extremely narrow and lithostratigraphic, units (Fig. 1D) (Khan et al., 1986; Grelaud cross-sectional taper (Davis and Lillie, 1994; Satyabala et al., et al., 2002). 2012). Jaumé and Lillie (1988) suggested that the northern The oldest unit is the Precambrian crystalline basement of the Potwar Plateau was initially a relatively high-angle (1.7°) taper Indian plate that crops out 80 km south of the Salt Range in the fold-thrust belt caused by frictional sliding on